The present application claims priority to Japanese Application(s) No(s). P2000-343634 filed Nov. 10, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a liquid crystal display element, and to a projection type liquid crystal display device which displays an image by using the liquid crystal display element.
2. Description of the Related Art
Conventionally, projection type liquid crystal display devices (liquid crystal projectors) which display an image by projecting light optically modulated by a liquid crystal display element (hereinafter referred to as a xe2x80x9cliquid crystal panel (LCD)xe2x80x9d) are known. The projection type liquid crystal display devices can adopt two image projecting methods, namely, a front projection method (front type) for projecting an image from the front side of a screen, and a rear projection method (rear type) for projecting an image from the rear side of the screen. Projection type liquid crystal display devices which produce color display are divided into a single-panel type using a single liquid crystal panel, and a three-panel type using three liquid crystal panels corresponding to light of three colors, red (R), green (G), and blue (B).
FIG. 9 shows the general configuration of an optical system (primarily, an illumination optical system) of a projection type liquid crystal display device as a related art. The projection type liquid crystal display device includes a light source 101, a pair of first and second multilens array integrators (hereinafter referred to as xe2x80x9cMLAsxe2x80x9d) 102 and 103, a PS beam combiner 104, a condenser lens 105, a field lens 106, a liquid crystal panel 107, and a projection lens 108 which are arranged in that order along an optical axis 100. The MLAs 102 and 103 have a plurality of microlenses 102M and 103M, respectively, arranged in a two-dimensional form. The PS beam combiner 104 includes a plurality of half-wave plates 104A arranged corresponding to the boundaries between the adjoining microlenses 103M of the second MLA 103.
In the projection type liquid crystal display device, illumination light emitted from the light source 101 passes through the MLAs 102 and 103, and is divided into a plurality of small beams. The light passed through the MLAs 102 and 103 enters the PS beam combiner 104 as light L10 including a P-polarized light component and an S-polarized light component which intersect each other in a plane perpendicular to the optical axis 100. The PS beam combiner 104 separates the light L10 into two types of polarized light components L11 and L12 (a P-polarized light component and an S-polarized light component, respectively). One of the separated polarized light component L11 emerges from the PS beam combiner 104 while maintaining its direction of polarization (for example, P-polarization direction). The other polarized light component L12 (for example, the S-polarized light component) emerges therefrom after being converted into another polarized light component (for example the P-polarized light component) by the action of the half-wave plate 104A. Consequently, the directions of polarization of the two separated polarized light components L11 and L12 are unified in a specific direction.
The light emerging from the PS beam combiner 104 passes through the condenser lens 105 and the field lens 106, and is directed onto the liquid crystal panel 107. The small beams separated by the MLAs 102 and 103 are enlarged to a magnification which is determined by the focal length fc of the condenser lens 105 and the focal length fML2 of the microlenses 103M in the second MLA 103, and illuminate the entire incident surface of the liquid crystal panel 107. Consequently, a plurality of enlarged beams are superimposed on the incident surface of the liquid crystal panel 107, and the entire incident surface is uniformly illuminated. The liquid crystal panel 107 spatially modulates the incident light according to image signals and emits the light. The light emerging from the liquid crystal panel 107 is projected onto a screen (not shown) by the projection lens 108, thereby forming an image on the screen.
In the liquid crystal panel, a thin-film transistor (TFT) and the like are formed as a driving device on the substrate, and therefore, a shielded region called a black matrix is formed between adjoining pixels. For this reason, the aperture ratio of the liquid crystal panel does not equal 100%. Conventionally, in order to increase the effective aperture ratio of the liquid crystal panel, for example, one or more light-collecting microlenses per dot (per pixel or per subpixel) are placed in the optical axis direction on a counter substrate disposed on the light incident side. Herein, the xe2x80x9ceffective aperture ratioxe2x80x9d of the liquid crystal panel refers to the ratio of light beams emerging from the liquid crystal panel to all light beams incident on the liquid crystal panel. In a projection type liquid crystal display device, in general, the effective aperture ratio of the liquid crystal panel is defined in consideration not only of the light loss of the liquid crystal panel, but also of the eclipse of light by the projection lens disposed on the downstream side.
FIG. 10 shows an example of a structure of the liquid crystal panel 107 using microlenses. For ease of viewing, a part of FIG. 10 is not hatched. The liquid crystal panel 107 includes a pixel electrode substrate 140B, and a counter substrate 140A placed opposed to the pixel electrode substrate 140B on the light incident side thereof with a liquid crystal layer 145 therebetween.
The pixel electrode substrate 140B includes a glass substrate 148, a plurality of pixel electrode portions 146, and a plurality of black matrix portions 147 placed on the light incident side of the glass substrate 148. The pixel electrode portions 146 and the black matrix portions 147 are arranged in a two-dimensional form. Each of the pixel electrode portions 146 is made of a conductive transparent material. Each of the black matrix portions 147 is formed between adjoining pixel electrode portions 146, and is shielded by, for example, a metal film. A switching element such as a TFT (not shown) is formed in each black matrix portion 147 so as to selectively apply a voltage to the adjoining pixel electrode portion 146 according to an image signal.
The counter substrate 140A includes a glass substrate 141, a microlens array 142, and a cover glass 144 arranged in that order from the light incident side. A resin layer 143 is formed between the glass substrate 141 and the microlens array 142. Although not shown, counter electrodes are interposed between the cover glass 144 and the liquid crystal layer 145 so as to generate a potential between the counter electrodes and the corresponding pixel electrode portions 146. The resin layer 143 is made of an optical resin having a refractive index n1.
The microlens array 142 includes a plurality of microlenses 142M made of an optical resin having a refractive index n2 ( greater than n1) arranged in a two-dimensional form corresponding to the pixel electrode portions 146. Each of the microlenses 142M is convex on the light incident side, and has a positive refractive power. The microlens 142M serves to collect light, which is incident thereon via the glass substrate 141 and the resin layer 143, toward the corresponding pixel electrode portion 146. As long as the projection lens 108 disposed on the downstream side has a sufficient F-number, light collected by the microlens 142M and entering an aperture 146A, of light incident on the liquid crystal panel 107, is available for image display. Such a microlens 142M allows more light to enter the aperture 146A of the pixel electrode portion 146 than in a case in which the microlens 142 is not formed. This increases the effective aperture ratio, and enhances the efficiency of light utilization.
Light 211, which enters the liquid crystal panel 107 with such a structure at a divergence angle xcex2 with respect to an optical axis 200 of the microlens 142M, is refracted by the power of the microlens 142M, and emerges therefrom while being diverged at a greater angle than in the case in which the microlens 142M is not used. In this case, a divergence angle xcex8 of the emergent light is the sum of an angle xcex1 produced by the power of the microlens 142M and the angular component xcex2, and satisfies a condition expressed by the following Equation (1):
xcex8=xcex1+xcex2xe2x80x83xe2x80x83(1)
When it is assumed that the focal length of the microlens 142M is designated fML and the maximum outer size (diameter) thereof is designated xe2x80x9c2axe2x80x9d, the angle xcex1 produced only by the power of the microlens 142M is defined by the following Equation (2):
tan xcex1=a/fMLxe2x80x83xe2x80x83(2)
The divergence angle (incident divergence angle) xcex2 of illumination light incident on the liquid crystal panel 107 is defined by the following Equation (3):
tan xcex2=rc/fcxe2x80x83xe2x80x83(3)
Where fc represents the focal length of the condenser lens 105 (FIG. 9), and rc represents the radius thereof.
When the divergence angle of the light emerging from the liquid crystal panel 107 is designated xcex8, a required F-number (Fno.) of the projection lens 108 is defined by the following Equation (4):
Fno.=1/(2 sin xcex8)xe2x80x83xe2x80x83(4)
In the above-described liquid crystal panel 107, when light having a great divergence angle xcex2 enters, it cannot be sufficiently collected in the aperture 146A by the lens action of the microlens 142M, and a part thereof is eclipsed by the black matrix portion 147. When the light emerges from the panel, it diverges at a greater angle by the power of the microlens 142M than in the case using no microlens, and the divergence angle xcex8 is increased, as shown in Equation (1). On the other hand, the projection lens 108 cannot take therein light having a divergence angle more than a predetermined angle which is determined by the F-number defined by Equation (4). For this reason, light having an excessively large emergent divergence angle xcex8 is eclipsed by the projection lens 108 disposed on the downstream side.
From the above, the incident divergence angle xcex8 must be decreased in order for the microlens 142M to improve the efficiency of light utilization. However, a decrease in the incident divergence angle xcex2 leads to an increase in the focal length fc of the condenser lens 105, as shown by Equation (3), and also to an increase in the focal length of the microlens 103M of the second MLA 103. Therefore, in order to decrease the incident divergence angle xcex2, the optical path length from the light source 101 to the liquid crystal panel 107 must be increased. Such an increase in optical path length enlarges the total size of the device, and decreases the efficiency of light utilization of the entire illumination optical system including the components disposed upstream from the liquid crystal panel 107. While the eclipse by the projection lens 108 can be avoided by using a lens having a large F-number as the projection lens 108 (for example, approximately 1.2 to 1.5) corresponding to the divergence angle xcex8, such a lens having a large F-number substantially enhances the design difficulty and increases the cost.
As described above, the illumination optical system and the microlenses 142M of the liquid crystal panel 107 have the following problems (i) to (iii):
(i) Light with a large incident divergence angle xcex2 undergoes eclipse at the black matrix portion of the liquid crystal panel or the projection lens.
(ii) By reducing the incident divergence angle xcex2, the effective aperture ratio of the liquid crystal panel is increased, but the efficiency of light utilization of the entire illumination system is decreased and the size of the device is increased.
(iii) The divergence angle xcex8 of light emerging from the liquid crystal panel is the sum of the angle xcex1 produced by the power of the microlens and the incident divergence angle xcex2, and the emergent light diverges at a greater angle than in the case in which the microlens is not used. For this reason, the projection lens must have a large F-number corresponding to a large divergence angle xcex8. This enhances the design difficulty of the projection lens and increases the cost.
The eclipse at the black matrix portion 147 described in (i) above can be reduced by decreasing the focal length of the microlens 142M of the liquid crystal panel 107. In this case, however, the angle xcex1 produced by the power of the microlens 142M increases, and the emergent divergence angle xcex8 also increases. This causes the above problem (iii). If the brightness is ensured by decreasing the F-number of the projection lens 108, the imaging performance is affected, and the size and manufacturing cost of the projection lens itself are increased. In an actual projection type liquid crystal display device, the length between the pixel aperture and the microlens is optimized by making the focal length fML of the microlens 142M long in accordance with the F-number of the projection lens 108. Therefore, the above problems (i) and (ii) remain unsolved.
As shown in FIG. 11, another type of liquid crystal panel has been proposed in which a microlens array 152 is also placed on the side of a pixel electrode substrate 140B, and the angle xcex1 produced by a microlens 142M of a microlens array 142 in a counter substrate 140A is nullified when the light emerges from the microlens array 152. The microlens array 142 in the counter substrate 140A is formed directly on the light emergent surface of a glass substrate 141. The other microlens array 152 is made of an optical resin, and is placed on the light emergent side of the pixel electrode substrate 140B. A glass substrate 151 is disposed on the light emergent side of the microlens array 152. The microlens array 152 includes a plurality of microlenses 152M corresponding to the microlenses 142M of the counter substrate 140A. Each microlens 152M is convex on the light emergent side, and has a positive power. The microlens 152M functions as a collimator in combination with the corresponding microlens 142M of the counter substrate 140A. In this liquid crystal panel, refractive indices n1 and n2 of the glass substrate 141 of the counter substrate 140A and the microlens 142M and refractive indices n3 and n4 of the microlens 152M of the pixel electrode substrate 140B and the glass substrate 151 satisfy a condition n2 greater than n1 and n3 greater than n4.
Light incident on this type of liquid crystal panel is first refracted at an angle xcex1 by the power of the microlens 142M of the counter substrate 140A, for example, as in incident light 212 shown in FIG. 11. The incident light 212 is then refracted at an angle of xe2x88x92xcex1, which is opposite from the angle xcex1, by the function as a collimator of the microlens 152M on the side of the pixel electrode substrate 140B. Consequently, when the light emerges from the microlens 152M, the angular component xcex1 produced by the power of the microlens 142M of the counter substrate 140A is nullified. Since the angular component xcex1 is nullified, the emergent divergence angle xcex8 equals xcex2 according to Equation (1), and can be made smaller by the angle xcex1 than in the type shown in FIG. 10. In this arrangement of the microlenses 152M, for example, when incident light 213 with a divergence angle xcex2 enters a microlens 152M-2 next to a microlens 152M-1, the microlens 152M-2 does not function as a collimator for the incident light. In this case, the above relationship xe2x80x9cxcex8=xcex2xe2x80x9d is disturbed, and the emergent divergence angle xcex8 is more than the incident divergence angle xcex2. This makes it impossible to increase the effective aperture ratio.
For example, Japanese Unexamined Patent Application Publication No. 5-341283 proposes a liquid crystal panel in which the incident divergence angle xcex2 is nullified when light emerges. The liquid crystal panel includes a pair of glass substrates and a liquid crystal layer disposed therebetween, and microlenses are arranged on both sides of at least one of the glass substrates corresponding to pixel apertures. The two microlenses disposed on both sides of the substrate have the same focal length, and the length therebetween is set to be equal to the focal length. For this reason, each of the microlenses has the optical property of collecting incident parallel light adjacent to a surface opposite from the substrate surface where the microlens is formed, thereby nullifying the incident divergence angle xcex2. In this liquid crystal panel, the microlenses are formed by an ion exchange method.
In the above publication, one surface of the microlens is convex toward the inner sides of the substrates, and the other surface (both end surfaces of the substrates) is flat. Moreover, the length between the microlens on the side of the pixel aperture and the pixel aperture is nearly zero. It is thought that the thickness of the substrate with the microlenses is approximately several tens of millimeters. In such a structure, however, it is quite difficult to produce the substrate having microlenses. In particular, in the production using the ion exchange method, it is difficult to adjust the thickness, and to precisely work a thin substrate having a thickness of approximately several tens of millimeters so as to achieve the desired optical properties of the microlenses. For example, while it is thought that there is a need to polish the surfaces of the microlenses disposed at both ends of the substrates, it is quite difficult to precisely polish such thin substrates. In recent years, there have been requests to increase the definition of liquid crystal panels and to decrease the pixel pitch. Consequently, more precise working is necessary. The liquid crystal panel disclosed in the above publication is disadvantageous in this respect.
The present invention has been made in view of the above problems, and an object of the invention is to provide a liquid crystal display element and a projection type liquid crystal display device in which the efficiency of light utilization is enhanced by increasing the effective aperture ratio without increasing the size and without enhancing the difficulty in production.
In order to achieve the above object, according to an aspect of the present invention, there is provided a liquid crystal display element including a liquid crystal layer, a pixel electrode portion having a plurality of pixel apertures for transmitting light, and at least one microlens array having a plurality of microlenses arranged in a two-dimensional form on at least one of a light incident side and a light emergent side of the liquid crystal layer corresponding to the pixel apertures. Each of the microlenses includes a light-collecting lens having at least one lens surface in the optical axis direction for collecting incident light toward corresponding one of the pixel aperture, and a field lens having at least one lens surface in the optical axis direction so that the focal position thereof substantially coincides with the principal point of the light-collecting lens. Both the light-collecting lens and the field lens may be formed on the light incident side of the liquid crystal layer, or, for example, the light-collecting lens may be formed on the light incident side of the liquid crystal layer and the field lens may be formed on the light emergent side.
Preferably, the focal position of the entirety of each of the microlenses substantially coincides with the pixel aperture. While it is thought that the vignetting factor becomes higher as the focal position of the entire microlens is placed closer to the pixel aperture, the vignetting factor is not always highest when the focal position completely coincides with the pixel aperture, in consideration of all the angular components of the incident light. For example, it is preferable to set the focal position so that the beam waist of the light coincides with the pixel aperture.
According to another aspect of the present invention, there is provided a projection type liquid crystal display device including a light source for emitting light, a liquid crystal display element for optically modulating incident light, and a projection lens for projecting the light modulated by the liquid crystal display element. The present invention is applied to the liquid crystal display element.
In the liquid crystal display element and the projection type liquid crystal display device of the present invention, the efficiency of light utilization is enhanced by increasing the effective aperture ratio without increasing the size and without enhancing the difficulty in production. Furthermore, for example, in a case in which incident light has a divergence angle component with respect to the optical axis, the divergence angle component is removed when the light emerges from the microlens array. Therefore, for example, even when the focal length of the microlens is reduced, the divergence angle of the emergent light is prevented from excessively increasing. This can reduce the eclipse of light by the projection lens used in, for example, the projection type liquid crystal display device.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.